Post-fabrication forging treatment

ABSTRACT

A method of modifying a grain structure of a metal article includes disposing at least a portion of a metal article in a molten salt bath in a pressure vessel. The molten salt bath has bath temperature that is below a melting point temperature of the metal article. The method also includes pressuring the molten salt bath in the pressure vessel to a forging pressure sufficient to cause pressure-driven grain changes in the metal article.

FIELD OF THE DISCLOSURE

The present disclosure is generally related to metallurgy, and in particular to post-fabrication forging treatment of metal articles.

BACKGROUND

Throughout the ages, many different metalworking techniques have been developed. Traditional examples of metalworking include casting, cold forging, hot forging, and machining. More recent examples of metal working include computer-controlled additive manufacturing techniques, such as selective laser sintering, selective laser melting, or binder jetting. “Casting” refers to placing molten metal in a mold and allowing the molten metal to cool and solidify in the mold to form an article. “Machining” refers to a process that is similar to sculpting, e.g., removing pieces of metal from a metal blank until what is left has a desired shape. “Forging” refers to shaping metal using pressure. In the case of a “cold forging” technique the metal is shaped using only pressure (e.g., without adding heat during shaping), whereas “hot forging” techniques apply pressure to the metal while the metal is heated to shape the metal.

Each of these techniques has benefits and limitations. Thus, when selecting a fabrication technique to form a metal article, many considerations come into play. Examples of such considerations include cost (including cost of materials, cost of equipment, cost of tooling, such as molds, dies, or bits, and cost of labor), when the article is needed, how many instances of the article are needed, mechanical properties that the article should have, dimensional and mechanical tolerances, as well as other factors. For example, if a casting mold for an article is available, it is often cheaper to cast the article than to machine or forge the article. As another example, a forging technique can be used to provide certain grain boundary characteristics that casting and machining generally cannot provide. As a further example, casting, machining, and computer-controlled additive manufacturing techniques can be used to generate complex shapes more readily than forging techniques.

As a result of such trade-offs, it can be challenging to select a fabrication technique for a particular metal article. For example, if only a single instance of a relatively simple article is needed, forging the article may be the cheapest and fastest option. However, if the article is more complex or needs to meet very strict tolerances, a computer-controlled additive manufacturing technique may be cheaper and/or faster than forging. Further, if many instances of the article are to be fabricated, the cheapest option (over the entire lifetime of the mold) may be to generate a mold and cast the articles, assuming casting can give the articles required mechanical properties. If casting cannot provide the required mechanical properties, then dies can be generated to forge the article using extrusion, drop forging, etc.

SUMMARY

In a particular implementation, a method of modifying a grain structure of a metal article includes disposing at least a portion of the metal article in a molten salt bath in a pressure vessel, where the molten salt bath is at a bath temperature that is below a melting point temperature of the metal article. The method also includes pressurizing the molten salt bath in the pressure vessel to a forging pressure sufficient to cause pressure-driven grain changes in the metal article.

In another particular implementation, a system for modifying a grain structure of a metal article includes a pressure vessel sized to receive the metal article, and a molten salt tank configured to store molten salt. The system also includes a first pump coupled to the molten salt tank and to the pressure vessel. The first pump is configured to provide the molten salt to the pressure vessel to at least partially submerge the metal article in a molten salt bath at a bath temperature that is below a melting point temperature of the metal article and to pressurize the molten salt bath to a forging pressure sufficient to cause pressure-driven grain changes in the metal article.

In a particular implementation, a forging pressure vessel includes one or more insulated walls defining an interior region and configured to withstand pressure within the interior region of at least five thousand pounds per square inch. The forging pressure vessel also includes one or more molten salt ports through the one or more insulated walls. The one or more molten salt ports are configured to transport molten salt into the interior region, out of the interior region, or both, to subject a metal article within the interior region to a molten salt bath at a bath temperature that is below a melting point temperature of the metal article and at a forging pressure sufficient to cause pressure-driven grain changes in the metal article. The forging pressure vessel further includes one or more quench medium ports through the one or more insulated walls. The one or more quench medium ports are configured to transport a quench medium into the interior region, out of the interior region, or both.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram that illustrates an example of a system for fabrication and subsequent forging treatment of a metal article.

FIG. 2 is a diagram that illustrates first examples of forging treatment of the metal article of FIG. 1.

FIG. 3 is a diagram that illustrates second examples of forging treatment of the metal article of FIG. 1.

FIG. 4 is a flow chart of an example of a method of forging treatment of the metal article of FIG. 1.

DETAILED DESCRIPTION

The present disclosure describes a post-fabrication forging techniques and systems that can be used to process metal articles to modify grain characteristics of the metal articles. For example, a casting, machining, computer-controlled additive manufacturing, or other techniques can be used to fabricate a metal article, and the post-fabrication techniques described herein can subsequently be used to modify grain characteristics of the metal article. Casting, machining, and computer-controlled additive manufacturing techniques can all be used to fabricate complex metal articles relatively quickly and inexpensively; however, metal articles fabricated using these techniques may have unacceptable mechanical properties for some applications. Many mechanical properties of metal articles are related to the grain characteristics of the metal. Thus, in the example above, changing the grain characteristics using the disclosed post-fabrication forging techniques can change the mechanical properties of the metal article.

Computer-controlled additive manufacturing techniques generally use layer-by-layer construction. For example, binder jetting is a process in which a binder (e.g., a polymer) is selectively applied to a layer of metal powder to bond the metal powder together in some regions. Subsequently, another layer of metal powder is applied over the first bonded layer and selectively bonded using the binder. After several bonded layers have been formed into the shape of an article, the article is baked to fuse particles of the metal powder together (e.g. to sinter the particles of the metal powder). This process forms a metal article that can have weaknesses at boundaries between particles, weaknesses between layers, and grain characteristics that limit strength and ductility of the article.

Selective laser sintering and selective laser melting use a layer-by-layer building process that is similar to binder jetting, but selective laser sintering and selective laser melting omit the binder. Instead, a laser is used to selectively heat portions of the metal powder to cause particles within the heated portions to be sintered together (in the case of selective laser sintering) or melted together (in the case of selective laser melting). Thus, articles manufactured using selective laser sintering and selective laser melting can have similar concerns as articles manufactured using binder jetting.

Post-fabrication heat treatment (e.g., in vacuum or in an inert gas environment) can be used to improve some characteristics of articles formed using computer-controlled additive manufacturing processes. For example, post-fabrication heat treatment of an article formed using selective laser sintering can encourage some additional grain growth, leading to increased density of the article. However, even after such heat treatment, articles formed using computer-controlled additive manufacturing processes tend to have characteristics (e.g., mechanical properties and grain characteristics) that are more similar to cast metal articles than to forged metal articles.

The post-fabrication forging treatment disclose herein uses heat and pressure to modify grain characteristics of a metal article. For example, the metal article can be disposed in a molten salt bath, and the molten salt bath can be pressurized to a forging pressure (e.g., 5000 pounds per square inch (psi) or more). The high temperature and high pressure of the pressurized molten salt bath used for the disclosed post-fabrication forging treatment refine and grow the grain structure of the metal article and exert a forging force. For example, grain boundaries of the metal article can be realigned, reshaped, merged, etc. by the heat and pressure. As a result, after the post-fabrication forging treatment, the metal article will have grain characteristics similar to grain characteristics that would result from fabricating the metal article using a hot forging process. To illustrate, after the post-fabrication forging treatment, the metal article will have grain boundaries that tend to be aligned with a surface geometry of the metal article, sometimes referred to herein as grain boundary alignment.

For comparison, articles that are fabricated using casting techniques tend to have randomly arranged grain boundaries. Similarly, articles that are fabricated using computer-controlled additive manufacturing processes tend to have grain boundaries that are randomly arranged or arranged in a manner that corresponds to layers of the layer-by-layer fabrication process. Additionally, metal articles fabricated by casting, machining, or additive manufacturing tend to be less dense than articles fabricated using forging techniques. The density and orientation of grain boundaries with surface geometry of the metal article that results from forging (and from the disclosed post-fabrication forging treatment) provides many of the mechanical benefits of forging. For example, the ductility and strength of forged articles (as compared to machined, cast, or additively manufactured articles) is, at least in part, attributable to the density and orientation of the grain boundaries due to forging.

In some implementations, equipment used for post-fabrication forging treatment (e.g., a forging pressure vessel) can also be used for other treatment processes, such as quenching or heat treatment. For example, a metal article can be disposed in a forging pressure vessel for a post-fabrication forging treatment process. Molten salt is added to the pressure vessel to submerge (or partially submerge) the metal article, and the molten salt is pressurized to a forging pressure. After a treatment period, the molten salt can be removed from the forging pressure vessel, and a quench medium (e.g., air, water, oil, etc.) can be added to the forging pressure vessel to quench the metal article. As another example, after a quenching treatment (in the forging pressure vessel or elsewhere), the metal article can be disposed in the forging pressure vessel and subject to an unpressurized or low-pressure heat treatment (e.g., molten salt or another heat source).

The temperature and pressure of the molten salt bath can be precisely and accurately controlled to achieve specified grain characteristics. For example, a post-fabrication forging treatment system can include a controller that controls and monitors the temperature and pressure of the molten salt bath and a duration of a forging treatment. When the post-fabrication forging treatment system also performs other operations, such as quenching or heat treatment, the controller can also control these other operations. To illustrate, the controller can control the temperature of a quench medium, the rate at which the quench medium is added to the forging pressure vessel, the quantity of the quench medium added to the forging pressure vessel, and the duration of the quenching operation. As another illustration, the controller can control the temperature of a heat treatment medium, the rate at which the heat treatment medium is added to the forging pressure vessel, the quantity of the heat treatment medium added to the forging pressure vessel, and the duration of the heat treatment.

In some implementations, the controller controls how long a forging operation or another operation lasts based on a timer (e.g., elapsed time since the operation started). In other implementations, data from one or more sensors can be used (alone or in conjunction with the timer) to control how long an operation lasts. For example, a forging operation can last for a specified time (e.g., 15 minutes) after the metal article or the molten salt bath within the forging pressure vessel reach a specified temperature (or pressure). In some implementations, the controller includes a memory device to store one or more process recipes that can be programmed by or selected by an operator. The process recipe(s) specify set points (e.g., temperature and pressure targets), durations, an order of operations, and other parameters that the controller uses to govern operation of the post-fabrication forging treatment system based on target grain characteristics.

Accordingly, the post-fabrication forging treatment systems and methods disclosed herein enable fabrication of forged articles using non-forging processes, such as casting, machining, or additive manufacturing. To illustrate, a non-forging process can be used for shaping (or coarse shaping) of the article, and subsequently a post-fabrication forging treatment can be used to modify grain characteristics of the article to be similar to grain characteristics of a forged article. This simplifies selection of a fabrication process because a fabrication process that is faster and/or cheaper than forging (such as casting, machining, or additive manufacturing) can be used to fabricate the article without sacrificing the benefits of mechanical properties that are associated with forging.

In the following, reference is made to features depicted in the drawings. In some of the drawings, multiple instances of a particular type of feature are used. Although these features are physically and/or logically distinct, the same reference number is used for each, and the different instances are distinguished by addition of a letter to the reference number. When the features as a group or a type are referred to herein (e.g., when no particular one of the features is being referenced), the reference number is used without a distinguishing letter. However, when one particular feature of multiple features of the same type is referred to herein, the reference number is used with the distinguishing letter. For example, referring to FIG. 2, grain structures of a metal article are illustrated and associated with reference numbers 208A and 208B. When referring to a particular one of these grain structures, such as a first grain structure 208A, the distinguishing letter “A” is used. However, when referring to any arbitrary one of these grain structure or to these grain structures as a group, the reference number 208 is used without a distinguishing letter.

As used herein, various terminology is used for the purpose of describing particular implementations only and is not intended to be limiting. For example, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Further, the terms “comprise,” “comprises,” and “comprising” are used interchangeably with “include,” “includes,” or “including.” Additionally, the term “wherein” is used interchangeably with the term “where.” As used herein, “exemplary” indicates an example, an implementation, and/or an aspect, and should not be construed as limiting or as indicating a preference or a preferred implementation. As used herein, an ordinal term (e.g., “first,” “second,” “third,” etc.) used to modify an element, such as a structure, a component, an operation, etc., does not by itself indicate any priority or order of the element with respect to another element, but rather merely distinguishes the element from another element having a same name (but for use of the ordinal term). As used herein, the term “set” refers to a grouping of one or more elements, and the term “plurality” refers to multiple elements.

As used herein, “coupled” can include “communicatively coupled,” “electrically coupled,” or “physically coupled,” and can also (or alternatively) include any combinations thereof. Two devices (or components) can be coupled (e.g., communicatively coupled, electrically coupled, or physically coupled) directly or indirectly via one or more other devices, components, wires, buses, networks (e.g., a wired network, a wireless network, or a combination thereof), etc. As used herein, “directly coupled” is used to describe two devices that are coupled (e.g., communicatively coupled, electrically coupled, or physically coupled) without intervening components. To illustrate, two components that are in direct physical contact with one another are “directly coupled” to one another.

FIG. 1 is a diagram that illustrates an example of a system 100 for fabrication and subsequent forging treatment of a metal article 104. The system 100 includes one or more fabrication devices 102 and a forging treatment system 101, including a forging pressure vessel 106, one or more pumps and tanks for materials used during treatment of the metal article 104, and a controller 124. Although the system 100 of FIG. 1 illustrates the fabrication device(s) 102, the fabrication device(s) 102 can be remote from a location of the forging treatment system 101. For example, the metal article 104 can be fabricated at a first location and moved to a second location for post-fabrication forging treatment. In some implementations, an owner or operator of the forging treatment system 101 can be distinct from the owner or operator of the fabrication device(s) 102. For example, the owner or operator of the forging treatment system 101 can purchase the metal article 104 from the owner or operator of the fabrication device(s) 102 and subsequently treat the metal article 104 using the forging treatment system 101. As another example, the owner of the metal article 104 (which can be the owner or operator of the fabrication device(s) 102 or a customer of the owner or operator of the fabrication device(s) 102) can contract with the owner or operator of the forging treatment system 101 for forging treatment services.

The fabrication device(s) 102 can fabricate the metal article 104 using any metal fabrication process or a combination of metal fabrication processes. For example, the fabrication device(s) 102 can include casting devices that cast the metal article 104. As another example, the fabrication device(s) 102 can include machining devices (e.g., a milling machine, cutters, grinders, polishers, etc.) that shape the metal article 104 from a blank. As yet another example, the fabrication device(s) 102 can include additive manufacturing devices (e.g., selective laser sintering devices, selective laser melting devices, binder jetting devices, etc.) that build the metal article 104 by fusing layers of metal particles. In still another example, the fabrication device(s) 102 can include devices that use a combination of different fabrication processes. For example, two or more machined parts can be fused together (e.g., welded) to form the metal article 104. Thus, the metal article 104 can be cast, machined, fused layer-by-layer (also referred to as “printed”), or assembled from two or more metal parts that are fused together. In some implementations, the metal article 104 (or portions thereof) can be forged. To illustrate, if the metal article 104 is formed of two or more parts that are welded together, the forging treatment system 101 can be used to modify grain characteristics of the weld. As another example, the metal article can be forged and subsequently machined to form a particular feature. In this example, the forging treatment system 101 can be used to modify grain characteristics of a machined portion of the metal article 104.

As explained above, the forging treatment system 101 includes the forging pressure vessel 106. The forging pressure vessel 106 includes one or more insulated walls 108. For example, if the forging pressure vessel 106 is spherical, the forging pressure vessel 106 can include a single insulated wall 108. In another example, if the forging pressure vessel 106 is cubical, the forging pressure vessel 106 includes six insulated walls 108. The insulated wall(s) 108 enclose a volume of space to define an interior region 110 that is sized and shaped to receive the metal article 104.

One or more ports extend through the insulated wall(s) 108 to enable introduction of various process media into the interior region 110 and extraction of the various process media from the interior region 110. For example, in FIG. 1, one or more molten salt ports 112 extend through the insulated wall(s) 108 to enable adding molten salt 136 to the interior region 110 to form a molten salt bath 146 in the forging pressure vessel 106. The molten salt port(s) 112 may also enable extraction of the molten salt 136 from the interior region 110 after a forging process is complete. FIG. 1 also illustrates one or more quench medium ports 118 extending through the insulated wall(s) 108 to enable adding a quench medium 142 to the interior region 110 to form a quench bath 164 in the forging pressure vessel 106. The quench medium port(s) 118 may also enable extraction of the quench medium 142 from the interior region 110 after a quenching process is complete.

When the quench medium 142 is a liquid (e.g., oil, water, a water/glycol solution, or brine), quenching the metal article 104 can liberate gases 168 due to combustion, evaporation, or both. To facilitate use of a liquid quench medium 142, the forging pressure vessel 106 can include a vapor control system 114 that includes one or more vapor ports 116. The vapor port(s) 116 are configured to vent from the forging pressure vessel 106 the gases 168 released by the quench medium 142.

The insulated wall(s) 108 are configured to retain heat (i.e., are insulated) to facilitate control of the temperature within the interior region 110. Further, the insulated wall(s) 108 are configured to withstand forging pressures that are applied to the metal article 104. Generally, the forging pressure is at least 5000 psi; however, even higher pressures can be used depending on the particular metal or alloy from which the metal article 104 is fabricated and depending on the goals of the forging treatment process (e.g., target grain characteristics 162 desired in the metal article 104 after the forging treatment). In some implementations, the forging pressure can be greater than or equal to 8000 psi; greater than or equal to 10,000 psi; greater than or equal to 12,000 psi; greater than or equal to 15,000 psi; greater than or equal to 17,000 psi; greater than or equal to 20,000 psi; or even greater than or equal to 22,000 psi. In various implementations, the insulated wall(s) 108 are configured to withstand any of these forging pressures.

In some implementations, one or more sensors are disposed within the forging pressure vessel 106. The sensor(s) can be coupled to connectors external to the insulated wall(s) 108 to provide sensor data signals to the controller 124 or other components (not shown), such as a fabrication data tracking system. For example, in FIG. 1, the sensor(s) include one or more temperature sensors 150 coupled to one or more first connectors 120 and one or more pressure sensors 152 coupled to one or more second connectors 122. In other implementations, the forging pressure vessel 106 includes fewer sensors (e.g., only the pressure sensor(s) 152), more sensors (e.g., a liquid level sensor or a chemical composition sensor in addition to the temperature and pressure sensors 150, 152), or different sensors (e.g., a liquid level sensor or a chemical composition sensor without the temperature or pressure sensors 150, 152).

The temperature sensor(s) 150 are configured to provide one or more temperature feedback signals 154 via the first connector(s) 120 to the controller 124. The temperatures sensor(s) 150 can include, for example, thermocouples, optical temperature sensors, temperature-sensitive diodes, thermistors, or other electrical or electronic devices that generate a signal having characteristics indicative of temperature or changes in temperature. The temperature sensor(s) 150 are configured to (e.g., positioned to or positionable to) monitor a temperature of the molten salt bath 146, a temperature of the quench bath 164, a temperature of the metal article 104, a temperature of a mold or die 148, or another temperature associated with the interior region 110. In some implementations, the temperature sensor(s) 150 include sensors to monitor two or more of the temperature of the molten salt bath 146, the temperature of the quench bath 164, the temperature of the metal article 104, the temperature of the mold or die 148, or another temperature associated with the interior region 110. In some implementations, the temperature sensor(s) 150 include one or more sensors to monitor a temperature difference within the interior region 110, such as difference between the temperature of the metal article 104 and the temperature of the molten salt bath 146.

The pressure sensor(s) 152 are configured to provide one or more pressure feedback signals 156 via the second connector(s) 122 to the controller 124. The pressure sensor(s) 152 can include, for example, an optical pressure sensor, a piezoelectric sensor, a capacitive sensor, or other electrical or electronic devices that generate a signal having characteristics indicative of pressure within the interior region 110, a pressure change within the interior region 110, or a difference in pressure between interior region 110 and outside the forging pressure vessel 106.

The vapor control system 114 can be active or passive. For example, if the vapor control system 114 is passive, the vapor port(s) 116 can be configured to (e.g., calibrated to) open responsive to a specific pressure within the forging pressure vessel 106. To illustrate, the force on the vapor port(s) 116 due to the pressure within the forging pressure vessel 106 can push the vapor port(s) 116 open. In this example, the vapor port(s) 116 can be manually reset after opening or can automatically reset responsive to the pressure within the forging pressure vessel 106 falling below a specific pressure. If the vapor control system 114 is active, the vapor port(s) 116 can be opened responsive to a control signal from the controller 124 or another control device, such as a controller of the vapor control system 114 or a safety system. The control signal can be sent responsive to value of the pressure feedback signal(s) 156 or based on an operation that is being performed. To illustrate, the vapor port(s) 116 can be opened at the beginning of a quench operation.

In FIG. 1, the forging treatment system 101 includes a molten salt tank 134 coupled to the forging pressure vessel 106 via a first pump 138. The molten salt tank 134 is configured to store the molten salt 136 (or a portion of the molten salt 136 that is not being used in the molten salt bath 146 at any particular time). In a particular implementation, the molten salt tank 134 includes or is coupled to a heater to control a temperature of the molten salt 136. In some implementations, the heater is also configured to melt a solid salt material to form the molten salt 136. In other implementations, the solid salt material can be melted elsewhere and disposed in the molten salt tank 134 in a liquid (e.g., molten) state. Although only one molten salt tank 134 is shown in FIG. 1, in other implementations, the forging treatment system 101 can include more than one molten salt tank 134. For example, a first molten salt tank can be used to store fresh or unused molten salt 136, and a second molten salt tank can be used to store used molten salt 136 extracted from the forging pressure vessel 106. In this example, the used molten salt 136 can be processed (e.g., filtered, reheated, etc.) before being returned to the fresh molten salt tank, or the used molten salt 136 can be disposed of. As another example, a first molten salt tank can be used to store a first type of molten salt 136 (e.g., molten sodium chloride), and a second molten salt tank can be used to store a second type of molten salt 136 (e.g., molten sodium nitrate). In this example, different types of molten salt 136 can be used depending on chemical compatibility with a metal or alloy of the metal article and the target grain characteristics 162.

The first pump 138 is coupled to the controller 124 to receive first control signals 158, and the controller 124 is configured to control operation of the first pump 138 using the first control signals. The first pump 138 is configured to (responsive to the first control signals 158) pump the molten salt 136 into the forging pressure vessel 106 from the molten salt tank 134 to form the molten salt bath 146 (e.g., to at least partially submerge the metal article 104). In some implementations, the first pump 138 is also configured to pressurize the molten salt bath 146 to the forging pressure after filling the interior region 110 with molten salt 136. In other implementations, such as when the molten salt bath 146 does not completely fill the interior region 110, the molten salt bath 146 can be pressurized using a pressurized gas (e.g., an inert gas, such as argon). In some implementations, the first pump 138 is also configured to extract the molten salt 136 from the forging pressure vessel 106 and return the molten salt 136 to the molten salt tank 134. In other implementations, the molten salt bath 146 can be dumped (e.g., driven by pressure and gravity) to extract the molten salt 136 from the forging pressure vessel 106.

In FIG. 1, the forging treatment system 101 also includes a quench medium tank 140 coupled to the forging pressure vessel 106 via a second pump 144. The quench medium tank 140 is configured to store the quench medium 142 (or a portion of the quench medium 142 that is not being used in the quench bath 164 at any particular time). Although only one quench medium tank 140 is shown in FIG. 1, in other implementations, the forging treatment system 101 can include more than one quench medium tank 140. For example, a first quench medium tank can be used to store fresh or unused quench medium 142, and a second quench medium tank can be used to store used quench medium 142 extracted from the forging pressure vessel 106. In this example, the used quench medium 142 can be processed (e.g., filtered) before being returned to the fresh quench medium tank, or the used quench medium 142 can be disposed of. As another example, a first quench medium tank can be used to store a first type of quench medium 142 (e.g., brine), and a second quench medium tank can be used to store a second type of quench medium 142 (e.g., oil). In this example, different types of quench media 142 can be used depending on chemical compatibility with a metal or alloy of the metal article and the target grain characteristics 162.

The second pump 144 is coupled to the controller 124 to receive second control signals 166. The second pump 144 is configured to (responsive to the second control signals 166) pump the quench medium 142 into the forging pressure vessel 106 from the quench medium tank 140 to form the quench bath 164 (e.g., to at least partially submerge the metal article 104). In some implementations, the second pump 144 is also configured to extract the quench medium 142 from the forging pressure vessel 106 and return the quench medium 142 to the quench medium tank 140. In other implementations, the quench bath 164 can be dumped (e.g., driving by pressure and gravity) to extract the quench medium 142 from the forging pressure vessel 106.

As illustrated in FIG. 1, the controller 124 includes one or more processors 126 and memory 128 (e.g., one or more memory devices). The memory 128 is accessible to the processor 126 and stores instructions 130 that are executable by the processor 126 to initiate or control one or more operations of the forging treatment system 101. For example, the processor 126 can execute the instructions 130 to process the temperature feedback signal(s) 154 and the pressure feedback signal(s) 156 to determine whether process conditions within the forging pressure vessel 106 correspond to respective set points as specified in one or more process recipes 132. The processor 126 can also execute the instructions 130 to generate the first control signals 158, the second control signals 166, other control signals (not shown, such as a control signal to the vapor control system 114), etc.

The process recipe(s) 132 specify process conditions for particular operations performed by the forging treatment system 101. For example, for a particular forging operation the process recipe(s) 132 can specify a bath temperature of the molten salt bath 146, a forging pressure, a type of molten salt 136 to be used, a forging time 160, a fill level of the molten salt bath 146, etc. As another example, for a particular quenching operation the process recipe(s) 132 can specify a type of quench medium 142 to be used, a quenching time (or a quenching endpoint, such as an endpoint temperature of the metal article 104), a fill level of the quench bath 164, etc. In some implementations, the process recipe(s) 132 can also specify other process parameters, such as a duration or temperature of a pre-heating operation performed before a forging operation, a delay between a forging operation and a quenching operation, or process conditions for other processes (besides forging and quenching) performed in the forging pressure vessel 106 (such as a duration and temperature of a heat treatment).

In an illustrative example of operation described below, processing the metal article 104 using the forging treatment system 101 includes a forging operation, a quenching operation, and a heat treatment operation. In other examples, the metal article 104 can undergo other processes using the forging treatment system 101, or one or more of the described operations can be repeated, omitted, or performed in a different order. The specific order of the operations, process conditions used in each operation, and the types of operations used will depend on the metal or alloy of the metal article 104 and the target grain characteristics 162.

In the illustrative example of operation, the metal article 104 is disposed in the forging pressure vessel 106. In some circumstances, as described further with reference to FIG. 3, the metal article 104 can be coupled to, placed on, or enclosed within a mold or die 148 that defines a target feature shape for the metal article 104. After the metal article 104 is disposed within the forging pressure vessel 106, an operator can start the forging process by selecting or programming a process recipe 132. The controller 124 subsequently initiates or controls operations performed by the forging treatment system 101.

The forging process includes pumping molten salt 136 into the forging pressure vessel 106 to at least partially submerge the metal article 104. After the metal article 104 is at least partially submerged in the molten salt bath 146 and before the molten salt bath 146 is pressurized, a pre-heating operation can be performed to gradually heat the metal article 104. For example, the bath temperature of the molten salt bath 146 is generally much hotter than room temperature. To illustrate, the bath temperature can be set to a forging temperature of the metal article 104. The forging temperature is below the melting point temperature of the metal of the metal article 104 but is hot enough that the metal begins to soften somewhat (e.g. greater than a recrystallization temperature of the metal article 104). As an example, the forging temperature can be greater than 500 deg. F. Suddenly exposing the metal article 104 to such a dramatic temperature change (e.g., from room temperature to the forging temperature) can cause shock. The pre-heating operation can be used to mitigate or avoid such shock. One way to perform the pre-heating operation is to heat the molten salt 136 in the molten salt tank 134 to a bath temperature that is lower than the forging temperature. The molten salt 136 is then pumped into the forging pressure vessel 106 to form the molten salt bath 146. The molten salt 136 can be circulated (e.g., by the first pump 138) between the forging pressure vessel 106 and the molten salt tank 134 to avoid cold spots (e.g., to achieve or maintain a relatively uniform temperature distribution within the molten salt bath 146). In this example, the temperature of the molten salt 136 in the molten salt tank 134 and the molten salt bath 146 can be gradually increased to the forging temperature.

After the molten salt bath 146 is at the forging temperature, the metal article 104 can be allowed to soak in the molten salt bath 146 for a period (e.g., a soak time). Soaking the metal article 104 in the molten salt bath 146 helps to achieve a relatively uniform temperature distribution within the metal article 104 (or within the portion of the metal article 104 that is submerged in the molten salt bath 146).

When a forging operation begins, the molten salt bath 146 is pressurized to a forging pressure. For example, the first pump 138 can pump molten salt 136 into the interior region 110 to fill and pressurize the molten salt bath 146. Alternatively, if the molten salt bath 146 does not completely fill the interior region 110, then a pressurized gas source (not shown) can be used to pressurize the interior region 110 and the molten salt bath 146. The metal article 104 remains in the molten salt bath 146 at the forging pressure for a duration indicated by the forging time 160. The duration of the forging time 160, the value of the forging pressure, and the bath temperature are selected based on the properties of the metal of the metal article 104 and the target grain characteristics.

While the metal article 104 is in the pressurized molten salt bath 146, the pressure and heat applied to the metal article 104 drive changes to the grain structure of the metal article 104. For example, a density of the metal article 104 can be increased due to grain growth. Additionally, grain boundaries can shift. For example, dislocations at the grain boundaries can consolidate and, to some extent, be reoriented (e.g., to align more closely with surface geometry of the metal article 104). The amount and type of pressure-driven grain changes that occur depend on the forging pressure, the bath temperature, and the forging time 160.

When the duration of the forging time 160 is complete, the molten salt bath 146 is extracted from the forging pressure vessel 106 (e.g., by the first pump 138). The metal article 104 can be removed from the forging pressure vessel 106, or can remain in the forging pressure vessel 106 for further treatment. For example, the metal article 104 can remain in the forging pressure vessel 106 for a quench treatment.

The quench treatment is performed by introducing the quench medium 142 into the forging pressure vessel 106 to form the quench bath 164 at least partially submerging the metal article 104. The goal of quenching is to control the rate of cooling of the metal article 104 after the forging operation to achieve particular grain characteristics and corresponding material properties of the metal article 104. The rate of cooling can be controlled based on the type of the quench medium 142 that is used. For example, quenching with air provides a slower cooling rate than quenching with water. When a liquid quench medium 142 is used, it is common for gases 168 to be liberated (e.g., due to evaporation and/or combustion). The vapor control system 114 vents the gases 168 from the forging pressure vessel 106 during the quench operation if needed.

The quench operation can end when a quench time is complete or based on temperature of the quench bath 164 or of the metal article 104. At the end of the quench operation, the quench bath 164 can be extracted from the forging pressure vessel 106 (e.g., by the second pump 144). The metal article 104 can also be removed from the forging pressure vessel 106, or can remain in the forging pressure vessel 106 for further treatment. For example, the metal article 104 can remain in the forging pressure vessel 106 for a heat treatment.

Heat treatment is performed to relieve some of the internal stresses introduced into the metal article 104 due to the quenching operation. The heat treatment is performed by introducing molten salt 136 into the forging pressure vessel 106 to at least partially submerge the metal article 104 in a molten salt bath 146. The heat treatment can use the same type of molten salt 136 that was used for the forging treatment or a different type of molten salt 136 can be used. Additionally, the bath temperature of the molten salt bath 146 used for the heat treatment can be the same as the bath temperature used for the forging treatment (e.g., the forging temperature) or the bath temperature can be different (e.g., lower) during the heat treatment than during the forging treatment. For example, the forging temperature can be greater than a recrystallization temperature of the metal article 104 and the heat treatment can be performed at a temperature that is less than the recrystallization temperature of the metal article 104.

The heat treatment can end when a heat treatment time is complete or based on temperature of the molten salt bath 146 or of the metal article 104. At the end of the heat treatment, the molten salt bath 146 can be extracted from the forging pressure vessel 106 (e.g., by the first pump 138). The metal article 104 can also be removed from the forging pressure vessel 106, or can remain in the forging pressure vessel 106 for further treatment, such as cleaning to remove salt residue from the metal article 104. After a combination of the forging treatment, quench treatment, heat treatment, cleaning, and/or other processes, the metal article 104 has the target grain characteristics 162.

FIG. 2 is a diagram that illustrates first examples of forging treatment of the metal article 104 of FIG. 1. FIG. 2 illustrates a cross-sectional view 202 of an example of the metal article 104 at the beginning of the forging treatment (e.g., while the metal article 104 is submerged in the molten salt bath 146 and the molten salt bath 146 is pressurized to the forging pressure, as indicated by arrows 210). FIG. 2 also illustrates a cross-sectional view 204 of an example of the metal article 104 after the forging treatment.

As illustrated in the cross-sectional view 202, at the beginning of the forging treatment, the metal article 104 has a first grain structure 208A. In the illustrated example, the first grain structure 208A includes a plurality of relatively well-defined layers or grain boundaries. Relatively well-defined layers may be present, for example, in articles fabricated via additive manufacturing processes. In another example, relatively well-defined grain boundaries may be present in articles that are machined from forged blanks.

During or before the forging treatment illustrated in the cross-sectional view 202, the metal article 104 is heated to a forging temperature by the molten salt 136. The forging temperature is below the melting point temperature of the metal article 104, but heats the metal article 104 enough that the metal article 104 is slightly softened. During the forging treatment, the pressure of the molten salt bath 146 exerts a force on every surface of the metal article 104 that is exposed to the molten salt bath 146. In the cross-sectional view 202, the metal article 104 has a shape defined by the external surfaces 206 of the metal article 104, and each of the external surfaces 206 is exposed to the molten salt bath 146. Thus, the forging pressure exerts a forging force against each of the external surfaces 206. The forging force at each point on the external surface 206 has a component that is normal to the external surface 206 at that point.

The forging force (resulting from application of the forging pressure to the external surfaces 206) tends to press grains of the metal article 104 together, which can cause several changes to the grain characteristics. For example, some grains can merge, which tends to increase the density of the metal article. As another example, grains tend to flatten in a direction parallel to the forging force at each location and elongate in other directions. The flattening and elongation of the grains tends to realign grain boundaries to be roughly parallel with the external surfaces 206. Thus, in the cross-sectional view 204, the metal article 104 has a second grain structure 208B. In the second grain structure 208B, grain boundaries are more aligned with the external surfaces 206 of the metal article 104 than was the case for the first grain structure 208A. The second grain structure 208B can also be denser than the first grain structure 208A. As a result of the changes in the grain structure 208, the metal article 104 will tend to be stronger and more ductile after the forging treatment than before the forging treatment.

FIG. 3 is a diagram that illustrates second examples of forging treatment of the metal article 104 of FIG. 1. FIG. 3 illustrates a cross-sectional view 302 of an example of the metal article 104 at the beginning of the forging treatment (e.g., while the metal article 104 is submerged in the molten salt bath 146 and the molten salt bath 146 is pressurized to the forging pressure, as indicated by arrows 210). FIG. 3 illustrates a cross-sectional view 304 of an example of the metal article 104 after the forging treatment. In FIG. 3, the forging treatment illustrated uses a mold 148. For example, as illustrated in the cross-sectional view 302, the mold 148 has a circular cross-sectional shape that is sized to define a desired inner diameter of the metal article 104. The metal article 104 is coupled to or positioned with respect to the mold 148 before the forging treatment starts.

As illustrated in the cross-sectional view 302, at the beginning of the forging treatment, the metal article 104 has a first grain structure 308A. In the illustrated example, the first grain structure 308A is fairly random or disorganized. A random or disorganized grain structure may be present, for example, in articles fabricated via casting.

During or before the forging treatment illustrated in the cross-sectional view 302, the metal article 104 is heated to the forging temperature by the molten salt 136, as described with reference to FIG. 2. During the forging treatment, the pressure of the molten salt bath 146 is exerts a force on every surface of the metal article 104 that is exposed to the molten salt bath 146. In the cross-sectional view 302, the metal article 104 has a shape defined by an outer surface 306 and an inner surface 312. In the example illustrated, the inner surface 312 is adjacent to or in contact with the mold 148 and is not directly exposed to the molten salt bath 146; however, the outer surface 306 is exposed to the molten salt bath 146. Thus, the forging pressure exerts a forging force against the outer surface 306. At each point on the outer surface 306, the forging force has a component that is normal to the outer surface 306.

The forging force tends to press the metal article 104 against the mold 148 such that the shape of the metal article 104 (e.g., the shape of the inner surface 312), is conformed to a target feature shape defined by the mold 148. For example, after the forging treatment, the inner surface 312 has an inner diameter corresponding to an outer diameter of the mold 148. Further, as explained with reference to FIG. 2, the forging force tends to press grains of the metal article 104 together, which can increase the density of the metal article 104 and realign grain boundaries to be roughly parallel with the outer and inner surfaces 306, 312. Thus, in the cross-sectional view 304, the metal article 104 has a second grain structure 308B. In the second grain structure 308B, grain boundaries are more aligned with the outer surface 306 of the metal article 104 than was the case for the first grain structure 308A. The second grain structure 308B can also be denser than the first grain structure 308A. As a result of the changes in the grain structure, the metal article 104 will tend to be stronger and more ductile after the forging treatment than before the forging treatment.

FIG. 4 is a flow chart of an example of a method of forging treatment of the metal article of FIG. 1. For example, the method 400 or one or more operations of the method 400 can be performed by the system 100. In FIG. 4, the method 400 includes, at 402, fabricating a metal article using one or more of an additive manufacturing process, a casting process, or a machining process. For example, the fabrication device(s) 102 of FIG. 1 can fabricate the metal article 104 as explained with reference to FIG. 1. In some implementations, the forging treatment system 101 can be performed separately from the fabrication process, in which case, the method 400 can omit the fabrication of the metal article. For example, the metal article can be purchased or otherwise obtained rather than fabricated.

In FIG. 4, the method 400 also includes, at 404, disposing at least a portion of the metal article in a molten salt bath in a pressure vessel. The molten salt bath is at a bath temperature that is below a melting point temperature of the metal article. For example, the metal article 104 of FIG. 1 can be placed in the forging pressure vessel 106 and the first pump 138 can pump molten salt 136 from the molten salt tank 134 into the forging pressure vessel 106 to form the molten salt bath 146. The molten salt bath 146 at least partially submerges the metal article 104 in the forging pressure vessel 106.

The method 400 also includes, at 406, pressurizing the molten salt bath in the pressure vessel to a forging pressure sufficient to cause pressure-driven grain changes in the metal article. For example, the first pump 138 can pressurize the molten salt bath 146 to the forging pressure. The metal article 104 can remain in the pressurized molten salt bath 146 for a forging period (e.g., corresponding to the forging time 160) that is specified in a process recipe 132 and is selected to allow the metal article 104 to undergo pressure-drive grain changes (e.g., forging) sufficient to achieve the target grain characteristics 162.

In some implementations, the method 400 can also include post-forging processes. For example, in FIG. 4, the method 400 includes a quenching operation that includes, at 408, after maintaining the molten salt bath at the forging pressure for the forging period, disposing at least the portion of the metal article in a quench medium. Performing the quenching operation can include extracting the molten salt from the pressure vessel, at 410; introducing the quench medium into the pressure vessel, at 412; and venting from the pressure vessel gases released by the quench medium, at 414. For example, the first pump 138 can extract the molten salt 136 of the molten salt bath 146 from the forging pressure vessel 106. Subsequently, the second pump 144 can introduce the quench medium 142 into the forging pressure vessel 106 to form the quench bath 164. The gases 168 liberated when the quench medium 142 is introduced into the forging pressure vessel 106 can be vented by the vapor control system 114.

In the example illustrated in FIG. 4, the method 400 also includes a heat treatment process, which includes, at 416, after disposing at least the portion of the metal article in the quench medium, heat treating the metal article in the molten salt bath at a pressure less than the forging pressure. For example, the heat treatment process includes extracting the quench medium from the pressure vessel, at 418, and introducing molten salt into the pressure vessel, at 420.

The method 400 enables modifying a grain structure of a metal article after the metal article is fabricated. For example, the metal article can be fabricated using a fabrication technique that is selected based on cost, availability of equipment or tools, availability of materials, speed, efficiency (e.g., the amount of waste produced), availability of skilled labor, or any other relevant factor. Subsequently, the metal article can be treated via the method 400 to modify the grain structure of the metal article to have a target set of characteristics.

The illustrations of the examples described herein are intended to provide a general understanding of the structure of the various implementations. The illustrations are not intended to serve as a complete description of all of the elements and features of apparatus and systems that utilize the structures or methods described herein. Many other implementations may be apparent to those of skill in the art upon reviewing the disclosure. Other implementations may be utilized and derived from the disclosure, such that structural and logical substitutions and changes may be made without departing from the scope of the disclosure. For example, method operations may be performed in a different order than shown in the figures or one or more method operations may be omitted. Accordingly, the disclosure and the figures are to be regarded as illustrative rather than restrictive.

Moreover, although specific examples have been illustrated and described herein, it should be appreciated that any subsequent arrangement designed to achieve the same or similar results may be substituted for the specific implementations shown. This disclosure is intended to cover any and all subsequent adaptations or variations of various implementations. Combinations of the above implementations, and other implementations not specifically described herein, will be apparent to those of skill in the art upon reviewing the description.

The Abstract of the Disclosure is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, various features may be grouped together or described in a single implementation for the purpose of streamlining the disclosure. Examples described above illustrate but do not limit the disclosure. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present disclosure. As the following claims reflect, the claimed subject matter may be directed to less than all of the features of any of the disclosed examples. Accordingly, the scope of the disclosure is defined by the following claims and their equivalents. 

What is claimed is:
 1. A method of modifying a grain structure of a metal article, the method comprising: disposing at least a portion of a metal article in a molten salt bath in a pressure vessel, the molten salt bath at a bath temperature that is below a melting point temperature of the metal article; and pressurizing the molten salt bath in the pressure vessel to a forging pressure sufficient to cause pressure-driven grain changes in the metal article.
 2. The method of claim 1, further comprising, after maintaining the molten salt bath at the forging pressure for a forging period, disposing at least the portion of the metal article in a quench medium.
 3. The method of claim 2, wherein disposing at least the portion of the metal article in the quench medium comprises: extracting molten salt from the pressure vessel; and introducing the quench medium into the pressure vessel.
 4. The method of claim 3, further comprising venting, from the pressure vessel, gases released by the quench medium.
 5. The method of claim 2, further comprising, after disposing at least the portion of the metal article in the quench medium, heat treating the metal article in the molten salt bath at a pressure less than the forging pressure.
 6. The method of claim 5, wherein heat treating the metal article in the molten salt bath comprises: extracting the quench medium from the pressure vessel; and introducing molten salt into the pressure vessel.
 7. The method of claim 1, further comprising, before disposing at least the portion of the metal article in the molten salt bath, fabricating the metal article using one or more of an additive manufacturing process, a casting process, or a machining process.
 8. The method of claim 1, wherein the bath temperature is greater than a recrystallization temperature of the metal article.
 9. The method of claim 1, wherein the pressure-driven grain changes include grain boundary alignment.
 10. A system for modifying a grain structure of a metal article, the system comprising: a pressure vessel sized to receive a metal article; a molten salt tank configured to store molten salt; and a first pump coupled to the molten salt tank and to the pressure vessel and configured to provide the molten salt to the pressure vessel to at least partially submerge the metal article in a molten salt bath at a bath temperature that is below a melting point temperature of the metal article and to pressurize the molten salt bath to a forging pressure sufficient to cause pressure-driven grain changes in the metal article.
 11. The system of claim 10, further comprising a mold or a die defining a target feature shape for the metal article and configured to be coupled to the metal article while the metal article is at least partially submerged in the molten salt bath.
 12. The system of claim 10, further comprising: a controller coupled to the first pump, the controller including a processor and a memory, the memory storing instructions that are executable by the processor to cause the processor to send one or more first control signals to the first pump to control operation of the first pump.
 13. The system of claim 12, wherein the instructions define a process recipe associated with the metal article, wherein the process recipe specifies the bath temperature, the forging pressure, and a forging time based at least on a metal or alloy of the metal article.
 14. The system of claim 13, wherein the process recipe is further based on target grain characteristics of the metal article.
 15. The system of claim 12, further comprising: a quench medium tank configured to store a quench medium; and a second pump coupled to the quench medium tank and to the pressure vessel, the second pump configured to provide, responsive to one or more second control signals from the controller, the quench medium to the pressure vessel to at least partially submerge the metal article in a quench bath.
 16. The system of claim 15, further comprising a vapor control system coupled to the pressure vessel and configured to vent, from the pressure vessel, gases released by the quench medium.
 17. The system of claim 15, further comprising: one or more temperature sensors disposed in the pressure vessel and coupled to the controller to provide one or more temperature feedback signals to the controller; and one or more pressure sensors disposed in the pressure vessel and coupled to the controller to provide one or more pressure feedback signals to the controller.
 18. A forging pressure vessel comprising: one or more insulated walls defining an interior region and configured to withstand pressure, within the interior region, of at least five thousand pounds per square inch; one or more molten salt ports through the one or more insulated walls and configured to transport molten salt into the interior region, out of the interior region, or both, to subject a metal article within the interior region to a molten salt bath at a bath temperature that is below a melting point temperature of the metal article and at a forging pressure sufficient to cause pressure-driven grain changes in the metal article; and one or more quench medium ports through the one or more insulated walls and configured to transport a quench medium into the interior region, out of the interior region, or both.
 19. The forging pressure vessel of claim 18, further comprising: a temperature sensor disposed in the interior region and configured to provide a temperature signal to a first connector that is external to the one or more insulating walls; and a pressure sensor disposed in the interior region and configured to provide a pressure signal to a second connector that is external to the one or more insulating walls.
 20. The forging pressure vessel of claim 18, further comprising one or more vapor ports through the one or more insulated walls and configured to vent gases released by the quench medium. 